The invention relates to a process for the uphill low pressure casting of metal, particularly light metal, in that a casting mould provided with at least one feeder has its gate connected by means of a riser to a melting container and the melt contained therein is displaced under pressure through the riser into the casting mould.
In the past light metals, particularly aluminium, have acquired increasing significance as constructional materials. This also applies with respect to motor vehicle construction, particularly engine building. Thus, of late, engine blocks have been made from aluminium. Due to the mass production in the car industry it is also necessary to make available high efficiency casting processes and installations. In addition, a high quality standard must be maintained, particularly for heavy-duty components. As aluminium, particularly in the molten state, spontaneously oxidizes with atmospheric oxygen, in the presence of oxygen, an oxide skin very rapidly forms on open melt surfaces.
To avoid this to the greatest possible extent, e.g. both for mould casting and die casting, the low pressure casting process has proved satisfactory, particularly in implementing uphill casting, because the melt is not subject to turbulence and instead the mould or die is filled with a killed melt front. As a result it is possible to largely avoid oxide inclusions in the casting.
However, these measures typical for low pressure casting lead to the disadvantage that efficiency is relatively low. This can inter alia be attributed to the fact that after each casting or teeming operation, the pressure in the melting vessel must be reduced, which is in turn associated with the lowering of the melting column in the riser. Atmospheric oxygen penetrates the riser from the environment. Thus, an oxide skin is formed on the small surface of the melting column and during the further rising of said column during the next melting cycle is applied to the riser wall and is constantly re-formed on the melt front, so that the riser gradually becomes incrusted. Efficiency is reduced by the need to regularly replace the riser at relatively short time intervals. It is also an important disadvantage that the oxide skin formed on the surface of the melting column is introduced into the mould or die and subsequently reappears in the cast structure.
In low pressure casting installations it is known (WO 95/20449) to provide a closure in the area of the transition from the riser to the mould. Its main function is to prevent turbulence in the melting vessel, particularly in the gas cushion located above the melt level. The closure comprises a melting plate having a lower melting point than the aluminium melt and which is to be inserted at the transition between the riser and the gate. As the melt rises this closure plate is liquefied. These liquid foreign components are introduced into the mould and lead to highly undesired inclusions in the casting. It is necessary to replace the closure after each teeming process, so that efficiency is correspondingly low.
In another known process (WO 97/37797) between the riser opening and the mould is located a refeeder in the form of a container, through which the melt from the riser is displaced into the mould. The container has a bottom-side slide closure, which closes the mould after filling. Following teeming the mould together with the refeeder is uncoupled from the riser and the casting shrinkage is compensated by supplying melt from the refeeder into the mould. This also fails to make it possible to increase efficiency compared with conventional low pressure casting installations, particularly as the refeeder must be emptied for the next casting cycle and returned to the casting station.
According to another of applicants' (DE 198 07 623) at the end of each casting cycle the melting column extending from the riser into the mould gate is sheared close to the riser opening and accompanied by simultaneous closure thereof. As a result of the shearing of the melting column close to the riser opening, no free volume and therefore no free surface, where an oxide skin could form, is formed above the melting column. Thus, no oxide coating can become attached to the riser and cannot incrust the latter. The closure also ensures that no oxide skin is drawn into the mould during the following casting cycle. It is also advantageous if, following the shearing of the melting column, in the melting vessel is maintained an overpressure at least maintaining the melting column against the closure. This makes it possible to increase the efficiency of the casting installation.
The production of light metal castings in large numbers and high casting tonnages requires much higher specific casting capacities, which have hitherto been approximately 1 kg/s. To improve the economics of the installations, ever shorter cycle times are sought. Thus, there is also a demand for shorter casting times per cycle or for the same partial weights the requirement for a higher melt throughput during mould filling. The higher specific casting capacities of well above 1 kg/sec, e.g. 3 to 10 kg/sec in the case of uphill casting lead to high, local casting speeds. However, they are limited by the lack of erosion resistance of the moulding materials, through the geometry of the castings and ingates in narrow cross-sections, but mostly by the undesired, dynamic filling impact at the end of casting. For a given part geometry, dynamic filling impact at the end of casting is mainly dependent on the venting behaviour of the mould (gas permeability with sand moulds, vent holes, etc.) and in any case on the casting speed at the end of casting shortly before the complete filling of the mould.
In the sand casting process, particularly green-mould casting, surface faults on the castings are known, which are mainly caused by filling impact at the end of casting. In the final phase of mould filling the kinetic energy of the melt at the elevated mould surfaces is abruptly transformed into impact energy. This dynamic impact drives the melt into the pores of the sand surface and produces there undesired rough surfaces on the casting contour. As a result inhomogeneously compressed mould parts are affected to a varying extent. In many cases castings damaged in this way must be remachined with considerable dressing expenditure or must be abandoned as scrap.
Another effect of the filling impact is increased burr formation in the parting areas of the mould halves and the cores. Burr formation leads to expensive dressing activities, which as a function of the type of part can represent over 30% of the manufacturing costs of the casting. Therefore, great economic significance is attached to casting with little or no burr formation.
A third fault or defect type is blowhole formation due to air and gas inclusions, which arise during casting, but mainly in the phase just before the end of casting. As a preventive measure the casting gases are removed by means of air outlet ducts, vent holes, the use of open feeders or the use of moulding materials with a high gas permeability. Faults of the aforementioned type already occur with the presently conventional, low casting capacities of 1 to 2 kg/sec for aluminium alloys. If the casting capacity is significantly increased, it is to be expected that blowhole formation will significantly increase. Various uphill casting methods are known. Thus, e.g. a gas pressure cushion is used in the melting container or furnace and this fills the melt via the riser connected to the mould. In place of the gas pressure in the melting container use is also made of electromagnetic pumps at the foot of the riser and which take over the filling task. In both cases use is made of a so-called active filling, i.e. the filling process is controlled and regulated in the form of a speed-time profile or volume-time profile as a function of the level change in the melting container (DE 33 17 474=EP 0 128 280, EP 0 215 153). In principle, these methods make it possible to counteract the undesired, dynamic filling impact towards the end of mould filling. The increase in the casting tonnage and therefore the rise in the casting capacity per casting cycle make new demands on the known casting processes. Higher melt throughputs necessarily lead to larger melt or furnace charges and furnace volumes, because due to the high demands made on the melt quality the refilling cycles must not be too short. Large furnace volumes make the regulatability of the filling process more difficult, specifically in gas pressure filling processes. The greater mass inertia of the melt then leads in the case of rapid furnace pressure changes to oscillations of the metal mass in the furnace. This makes it more difficult or even impossible to control the melt filling profile. Particularly at the end of casting the previously accelerated melt mass cannot be slowed down sufficiently quickly and in oscillation-free manner.